Apparatus for the production of solidified trioxan

ABSTRACT

Apparatus for the preparation of solidified trioxan includes a cooled rotatable polished metal cylinder. A jacket partially surrounds the curved surface of the cylinder; and the jacket is preferably double walled for circulating a heating medium therethrough. An application is positioned to apply liquid trioxan to the surface of the cylinder where it solidifies and is subsequently removed therefrom.

United States Patent Decitre' Au 14 1973 [54] APPARATUS FOR THE PRQDUCTION 0F 2,465,489 3/ 1949 501ml 260/340 S D F T N 2,654,125 10/1953 Hall 264/212 I 2,774,988 12/1956 Stim et al. 425/447 X [75] Inventor: Pie e eci tre, Bethun an e 2,827,659 3/1958 Rabun 425/374 x [73] Assignee: Houllleres du Bassln du Nord & du T at pudecaws Nerd. France [22] Filed: May 1971 Primary Examiner-Richard B. Lazarus 21 App] 43, 47 Attorney-Ward, McElhannon, Brooks & Fitzpatrick Related U.S. Application Data [62] Division of Set. NO. 675,835, 06!. 17, 1967, Pat. NO. AB RACT 3590! Apparatus for the preparation of solidified trioxan in- 4 4 9 cludes a cooled rotatable polished metal cylinder. A 5 425/224 425/4 jacket partially surrounds the curved surface of the cyl- 58 Fieid 471 223 inder; and the jacket is preferably double walled for cir- I224 culating a heating medium therethrough. An application is positioned to apply liquid trioxan to the surface [56] References Cited of the cylinder where it solidifies and is subsequently I UNITED STATES PATENTS remm'ed he'efmm 383,174 5/1888 Fuchs 425/447 X 6 Claims, 4 Drawing Figures Pmmnted Augw 14, 1973 3,752,624

APPARATUS FOR THE PRODUCTION OF SOLIDIFIED TRIOXAN This application is a division of my copending U.S. Pat. application, Ser. No. 675,835, filed Oct. 17, 1967 now U.S. Pat. No. 3,590,110.

The present invention is concerned with a process for the production of solidified trioxan and with apparatus for carrying out the process.

Trioxan prepared by the polymerisation of formaldehyde is obtained, after purification, at a relatively high temperature in the form of a liquid having a very high vapour pressure. At room temperature trioxan is solid and it is, as a rule, stored and used in that form. However, since trioxan is very sensitive to impurities and has a very high vapour pressure, the operation of solidifying liquid trioxan is subject to considerable difficulties. Thus, there is no apparatus currently available which can be used for the continuous preparation of homogeneously crystallised relatively thick sheets of pure trioxan; at present a semi-continuous method is used in which the trioxan is crystallised in separately cooled tanks. However, this method is itself unsatisfactory since it gives an inadequate yield for the preparation of large quantities of solid trioxan and the crystalline structure of the resulting product is generally inhomogeneous.

We have' now developed a continuous process for solidifying and crystallising liquid trioxan and, according to the present invention, we provide a process which comprises continuously applying liquid trioxan at a temperature of from 62 to 70 C on to a cooling surface maintained at a temperature of from to 55 C, the surface being moved relative to the point of application of the liquid trioxan so that the trioxan which has solidified on the surface is not in contact with a considerable excess of hot liquid trioxan, and continuously removing the solidified trioxan from the surface.

By means of this process, sheets of solidified trioxan can be obtained in which the trioxan crystals have satisfactory dimensions and an identical orientation (the main axes of the crystals being perpendicular to the surface of the sheet). The cooling surface is advantageously the curved surface of a rotating polished metal cylinder; the liquid, trioxan is preferably applied adjacent the upper or top generator of the cylinder and can be applied at other points on the surface of the cylinder, particularly adjacent the lower or bottom generator. Excess liquid trioxan, i.e., trioxan which does not solidify on the surface, is preferably r'e-cycled.

A considerable difficulty encountered in the crystallisation of trioxan is the fact that the solidified trioxan tends to adhere to the cooling surface to which it is applied. It istherefore very advantageous to employ a crystallisation method in which the solid trioxan can be readily detached from the cooling surface. This is surprisingly achieved in the process according to the in- The temperature of the liquid trioxan poured on to the cooling surface is from 62 to C. Trioxan melts at 62 C; the upper limit of 70 C is set by the likelihood of the trioxan being contaminated with various impurities at higher temperatures and by the rate of heat transfer between the trioxan and the cooling surface in relation to the crystallisation speed of the trioxan.

The present invention also includes apparatus for carrying out the above-described process, the apparatus comprising a rotatable polished metal cylinder provided with cooling means, a jacket surrounding the curved surface of the cylinder, the jacket having side walls which bear against the curved surface of the cylinder, and means positioned between the jacket and the cylinder for applying liquid trioxan to the curved surface of the cylinder.

The jacket is preferably double walled and provided with means for circulating a heating medium therethrough. The inner edges of the side walls of the jacket are preferably provided with sealing elements which bear against the curved surface of the cylinder, the sealing elements being provided with heating means.

In aparticula r embodiment of the apparatus, the solidified trioxan is detached from the cylinder and broken into uniformly sized small sheets by the combination of a cylinder which bears against the solidified trioxan on the cooling cylinder and a curved metal element extending across the width of the cylinder, this element being shaped to detach the solidified trioxan from the cooling cylinder and to deliver it to a suitabl positioned conveyor belt.

The diameter of the cooling cylinder and its speed of rotation are also important factors: in obtaining homo geneous solid trioxan. These parameters also affect the rate of production of solid trioxan since the larger the cylinder and the faster it rotates, the greater the pro duction of solid trioxan. The trioxan must have an adequate crystallisation time and the speed of rotation of the cylinder must be adapted to its diametenFor example, the speed of rotation of a cylinder having a radius of 14 cm is suitably from one-third and one-tenth of a revolution per minute. i

In order that the invention may be more fully understood, suitable forms of apparatus for carrying out the process, and their methods of operation, will now be described, by way of example only, with referece to the accompanying drawings in which:

FIG. 1 is a diagrammatic end elevation of a first apparatus;

FIG. 2 is a diagrammatic sectional elevation of a second apparatus;

FIG. 3 is a diagrammatic side elevation, partly in section, of the apparatus of FIG. 2; and

FIG. 4 is a diagrammatic end elevation of solidified trioxan collecting means.

Referring to FIG. 1, the apparatus comprises an internally water' cooled rotatable stainless steel cylinder C and a delivery pipe T for liquid trioxan spaced a distance x from the top generator of the cylinder C; the delivery pipe can be reciprocated across the width of the cylinder. Positioned below the cylinder C is a semicylinderical collecting trough provided with an outlet for excess liquid trioxan. During operation a layer of solidified trioxan having a thickness e is formed on the cylinder and is detached therefrom at the point D.

A number of runs were carried out using apparatus as shown in FIG. 1 and these are described in Example 1.

EXAMPLE 1 TABLE 1 Recycle rate Speed olThickness Distance Rate of supply at in cm. of liquid r in g/min. rotation e in mm.

trioxan a r.p.m. in glmin. 2.5 l 18 48 l/6 5 2.5 280 I50 l/6 5.3 2.5 460 360 l/6 4.5 200 60 U6 5.3 265 I l/6 5.9 0 265 I05 I13 4.] 0 265 I05 2/3 3 l l 190 90 H6 4 The following facts were ascertained from these runs: When the ratio r/a (ratio between the recycle rate and the supply) is lower than 0.4, the solid trioxan obtained is not uniform and its surface has rather large crystallites; when 0.4 r/a 0.5, the trioxan layer formed becomes continuous and more regular, but its outer surface looks bumpy and its internal surface has a crystalline structure different from the crystalline structure of the body; when r/a 0.5, the faults mentioned above disappear and when the resulting solid trioxan is polymerised after irradiation, a crude polyoxymethylene of homogeneous structure can be obtained.

Although the apparatus of FIG. 1 can be used for carrying out the process according to the invention, it has a number of disadvantages, as follows: there is difficulty in supply via the delivery pipe T leading to a band of trioxan of rather irregular thickness; the unprotected hot trioxan is easily contaminated; the liquid trioxan has to be recycled at a high rate; there is a considerable loss of trioxan due to its high vapour pressure and there is the risk of fouling the ends of the cylinder.

A preferred form of apparatus is shown in FIGS. 2 and 3. This apparatus comprises a rotatable internally water cooled cylinder C of molybdenum-containing 18,8 stainless steel and a souble walled jacket E which substantially completely surrounds the curved surface of the cylinder but leaves its ends open to the air (as shown in FIG. 3). Means (not shown) are provided for circulating a heating medium through the jacket E and the side walls of the jacket are provided with hollow sealing elements T (see FIG. 3) through which a heating medium can be passed.

Opening into the top of the jacket E is a delivery pipe for liquid trioxan which leads to a trough positioned between the jacket E and the top of the cylinder C and extending across the width of the jacket. Positioned in the trough, and extending along its length, is a rotatable impeller M When the trough contains liquid trioxan and the impeller is rotated, the vanes of the impeller project the liquid trioxan on to the top of the cylinder adjacent the top generator thereof. The bottom of the jacket E forms another trough in which is positioned a second impeller M when excess liquid trioxan collects at the bottom of the jacket and the impeller M, is rotated, its vanes project the liquid trioxan on to the cylinder adjacent the lower generator thereof.

In operation the jacket E is maintained at a temperature such that trioxan vapour cannot condense on its inner surface and the sealing elements T are maintained at a temperature 2 or 3 C higher than that of the jacket T. The band of solidified trioxan is formed, at a controlled temperature, only on the curved surface of the cylinder without the ends of the cylinder becoming dirtied; trioxan vapour cannot condense on the internal surface of the jacket since it is always kept at an adequate temperature; the band of solidified trioxan is insulated from the side walls of the jacket by a thin film of molten trioxan which forms in contact with the sealing elements T whose temperature is controlled. This device substantially eliminates friction and adhesion between the band of solid trioxan and the edges of the side walls of the jacket and thus prevents the premature tearing away of the solid trioxan from the surface of the cylinder.

A series of runs were carried out using apparatus as shown in FIGS. 2 and 3 and these are described in Example 2.

EXAMPLE 2 In carrying out the runs, the surface temperature of the cylinder C was maintained at 23 C. The cylinder had a diameter of 28 cm. and it was rotated at rates varying from to 1/6 of a revolution per minute. The liquid trioxan was delivered at a temperature of 64 C.

The variations in the operating conditions and the results obtained are shown in Table 2.

TABLE 2 Recycle Rate of supply rate, Speed of Thicknes of liquid trioxan r in rotation e of solid a mg./min. g/min. in r.p.m. trioxan.

in mm. I40 16 No 4.8 20 H3 3.4

The best results, as regards both the external appearance of the band of solid trioxan and its homogeneity of crystallisation, were obtained with a ratio r/a of from 0.05 to 0.15. The use of the apparatus shown in FIGS. 2 and 3, as compared with that shown in FIG. 1, makes the supply of liquid trioxan to the cylinder much more homogeneous and the band of solid trioxan has a highly uniform thickness; the trioxan is no longer contaminated and the recycling rate of liquid trioxan is low.

If the solid trioxan is to be collected in the form of individual small sheets of substantially constant width, the apparatus shown in FIGS. 2 and 3 is provided with an extra device as shown in FIG. 4.

Referring to FIG. 4, the apparatus additionally comprises a rotatable stainless steel cylinder C,, the spacing of which from the cylinder C can be varied and which is adapted to bear against the band of solidified trioxan on cylinder C at point A; a conveyor belt BT, suitably of rubberised canvas, which passes over a roller r, which is suitably faced with stainless steel; and a guide blade G formed from a sheet of stainless steel and disposed as shown in the figure. The guide blade extends across the width of the cylinder G and its edge adjacent the latter, which is preferably tapered, is positioned close to, but not in contact with, the surface of the cylinder; the guide blade G cannot, therefore, scratch the surface of the cylinder, but its edge is close enough to the surface to deflect the band of solid trioxan from the cylinder C.

In operation, pressure is applied to the solidified trioxan by the cylinder C which is rotated so that its linear peripheral speed is the same as that of the band of trioxan, and when the band of solid trioxan comes into contact with the edge of the guide G, the band is deflected from the cylinder C and breaks at the level A. On further rotation, the detached sheet of solid trioxan is moved upwards and progressively outwards from the cylinder C until it falls onto the conveyor BT. Referring to FIG. 4, the individual sheets pass progressively from position 1 to positions 2 and 3.

The operation of the process according to the invention using the apparatus described enables a band and- /or small sheets of solid trioxan to be obtained of high purity and high crystalline homogeneity throughout. The thickness of the trioxan band can be regulated as required, it being preferred to obtain a thickness of from 2 to 8 mm.

The solid trioxan thus prepared can be packed and sold for all uses. It can be directly used as a monomer for the preparation of polyoxymethylene by known methods, more particularly by irradiation and polymerisation in the solid phase, the outstanding crystalline homogeneity of the solid trioxan enabling homogeneous polyoxymethylenes of very high molecular weight to be obtained in a readily reproducible manner.

I claim:

1. Apparatus for the production of solidified trioxan which comprises, a rotatable polished metal cylinder provided with cooling means, a jacket partially surrounding the curved surface of the cylinder and spaced therefrom, said jacket having side walls terminating closely adjacent to the curved surface of the cylinder, said side walls including sealing elements which bear against the curved surface of said cylinder said sealing elements being provided with heating means, delivery means positioned between the jacket and cylinder for applying liquid trioxan to the curved surface of the cylinder, and means for removing solidified trioxan from said cylinder. r

2. Apparatus according to claim 1 wherein said jacket is double walled for circulation of a heating medium.

3. Apparatus for the production of solidified trioxan which comprises, a rotatable polished metal cylinder provided with cooling means, a jacket at least partially surrounding the curved surface of said cylinder and spaced therefrom, said jacket having side walls terminating closely adjacent to the curved surface of the cylinder, a trough positioned within said jacket and extending across the width thereof, a delivery pipe extending through said jacket and positioned to deliver liquid trioxan to said trough, an impeller positioned in said trough to distribute said liquid trioxan to the surface of said cylinder, and means for removing solidified trioxan from said cylinder.

4. Apparatus according to claim 12 wherein said side walls include sealing elements which bear against the curved surface of the cylinder, said sealing elements being provided with heating means.

5. Apparatus according to claim 3 wherien said jacket is dimensioned to form a second trough for collecting excess liquid trioxan, and a second impeller is positioned in said second trough to project said excess liquid trioxan to the surface of said cylinder.

6. Apparatus according to claim 3 wherein said jacket is doubled walled for circulation of a heating medium.

' UNITED STATES PATENT OFFICE i CERTIFICATE OF CORRECTION Patent No. q 752 I624 Dated August 14, 1973 InventorQiQQ Pierre Decitre It is certified that error appears in the aboveidentified patent and that said Letters Patent are hereby corrected as shown below:

Col. 3, line 55,"souble" should be -'-double-.

Col 6, line 25,"12" should be --3--.

Enter on page 1, [30] Foreign Application Priority Data October 17', 1967 France... ..67583 5/ 67 Signed and sealed this 17th day of September 197 (SEAL) qAttest:

a c. MARSHALL DANN 'MCCOY M. GIBSON, JR.

Attesting Officer Commissioner of Patents 

2. Apparatus according to claim 1 wherein said jacket is double walled for circulation of a heating medium.
 3. Apparatus for the production of solidified trioxan which comprises, a rotatable polished metal cylinder provided with cooling means, a jacket at least partially surrounding the curved surface of said cylinder and spaced therefrom, said jacket having side walls terminating closely adjacent to the curved surface of the cylinder, a trough positioned within said jacket and extending across the width thereof, a delivery pipe extending through said jacket and positioned to deliver liquid trioxan to said trough, an impeller positioned in said trough to distribute said liquid trioxan to the surface of said cylinder, and means for removing solidified trioxan from said cylinder.
 4. Apparatus according to claim 12 wherein said side walls include sealing elements which bear against the curved surface of the cylinder, said sealing elements being provided with heating means.
 5. Apparatus according to claim 3 wherien said jacket is dimensioned to form a second trough for collecting excess liquid trioxan, and a second impeller is positioned in said second trough to project said excess liquid trioxan to the surface of said cylinder.
 6. Apparatus according to claim 3 wherein said jacket is doubled walled for circulation of a heating medium. 